Prostate cancer is typically diagnosed by transrectal US guided needle biopsy. However, due to the low sensitivity of this procedure, repeat biopsies are often required to confirm a diagnosis. It is known that using additional imaging information from Magnetic Resonance (MR) can improve the sensitivity of the biopsy procedure, due to the additional subsurface detail that is offered by MR compared to US images. For example, MR findings of suspected cancer nodules can be used to guide needle placement during US-guided biopsy.
In order for MR to be effectively used to supplement US guidance, an image registration procedure is required to fuse the MR and US images into a common coordinate space. In clinical practice, this image registration is often merely done mentally by the clinician after consulting both images visually, although there are a number of techniques to perform this registration computationally. The simplest method involves computing a rigid transformation to align the prostate using point or surface features. However, a rigid transformation is typically insufficient to accurately align prostate structures due to soft tissue deformation which occurs during US imaging. More sophisticated methods utilize some kind of deformable registration, such as a biomechanical tissue model.